So she began picking. First, there was a load of figs, which she intends to make into jam for a cafe that feeds homeless people. Then, for herself, she got five pounds of tomatoes, two kinds of squash and — the real prize — a Sugar Baby watermelon.

“I don’t think of it as stealing,” she said. “These things were planted by a person who was going to harvest them. That person no longer has the ability to. It’s not like the bank people who sit in their offices are going to come out here and pick figs.”

Of course, a police officer who catches her might not agree with Ms. Callahan’s legal assessment. And it would be a rare bank official who would sign off.

But as the world of urban fruit and vegetable harvesting grows, the boundaries around where to grow and pick produce are becoming more elastic.

Over the last few years, in cities from Oakland, Calif., to Clemson, S.C., well-intentioned foraging enthusiasts have mapped public fruit trees and organized picking parties. Volunteers descend on generous homeowners who are happy to share their bounty, sometimes getting a few jars of preserves in return.

There are government efforts to turn abandoned land into food, too. In Multnomah County, Ore., officials offer property that has been seized for back taxes to community and governmental organizations for gardens.

But with more and more properties in foreclosure and large stretches of vacant lots available in some cities, a new, guerrilla-style harvest is taking shape.

Robby Astrove works with Concrete Jungle, a fruit-foraging organization in Atlanta that in 2009 began building a database of untended fruit and nut trees on commercial and public land. The group donates most of the food to agencies that feed the hungry.

Although Mr. Astrove and his colleagues have harvested abandoned community gardens and he has planted pear and fig trees on empty commercial property, the organization cautions volunteers against trespassing and does not pick fruit on foreclosed properties.

Still, he thinks it is a great idea, especially for cities like Atlanta, where one in 50 homes is in foreclosure. Already, he said, there is an underground network among the homeless who work the gardens and trees around vacant homes, he said.

“It’s a perfect storm of vacant properties and people who need a quality food source and an unused resource,” Mr. Astrove said.

One of the best-known urban foragers is Anna Chan, who lives in Clayton, Calif., east of San Francisco. She is called the Lemon Lady and was recently featured in People magazine.

Three years ago, Ms. Chan began collecting fruit that was going uneaten and delivering it to food banks. She soon expanded her efforts to local farms and grocery store produce departments. Since then, she and a group of volunteers have delivered more than 250 tons of fruits and vegetables to the hungry, she said.

But she has never harvested on foreclosed or abandoned property.

“I try to promote the legal way,” she said. “Without permission, it’s tricky. It’s trespassing.”

But she, too, applauds people like Ms. Callahan.

“It’s a beautiful idea,” she said. “It doesn’t matter if it’s a neighbor’s tree or a vacant lot or a foreclosure or whatever. It’s you and that fruit tree right at that precise moment when the fruit is ready and you need to make something happen.”

The point, she and other urban fruit foragers say, is to keep food from going to waste. Ms. Callahan, who works for the Carter Center and lived in Africa for eight years, has seen true hunger and cannot bear to watch food rot.

“If food is going bad on the vine,” she said, that says something about us as a society. “It doesn’t matter if the bank owns it. We should be more communal than that.”

Although urban foragers see no harm in picking the produce, one would be hard-pressed to find a real estate agent or a banker who would officially encourage the practice.

Still, a ripe fig is a ripe fig.

“If I lived next to somebody who had abandoned fruit trees, I’d go get some myself,” said Jim B. Miller Jr., the chairman of Fidelity Bank in Atlanta. “You shouldn’t be starting a garden on somebody’s property, and you can carry this too far, but if there’s fruit on that tree, it ought to be eaten.”

This article has been revised to reflect the following correction:

Correction: August 15, 2011

An earlier version of this article included a picture that was published in error. The home shown in the photo is occupied; it is not among the vacant homes being foraged for fruit and vegetables.